The Over-The-Horizon Radars (OTHR) use the refraction of the waves on the ionosphere to reach the targets over the horizon. In order to know the real position of the targets the actual ionospheric characteristics must be estimated. So an inversion method to obtain them has been developed. This inversion method is capable to obtain the parameters of a model of the electronic density profile from some measurements realized with the radar, the backscatter ionograms. To optimize the method two algorithms has been introduced: the simulated annealing and the genetic algorithm.